Michael R. Stern 
The Portal At The End Of The Storm [EPUB ebook] 

Fritz Russell has disappeared into the portal, leaving a trail impossible to follow. This has caused a chain reaction that ripples across the time-space continuum.
Undaunted, his friend Ashley searches for Fritz, determined to find him and reverse the disruption of time. Attempting to undo the changes Fritz has done, Ashley discovers a shocking connection between parallel universes and the portal.
Disturbing the flow of time comes with a heavy price. But will it prevent him from bringing Fritz home?